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Introduction

Cynthia Goldsmith/CDC

Zika virus is an infectious agent that causes an illness in humans known as Zika fever. Zika virus is closely related to the viruses that cause dengue, yellow fever, and West Nile disease. Like those diseases, Zika is spread by the bite of infected mosquitoes.

Zika virus was first isolated in 1947 from a monkey captured in the Zika Forest in Uganda. The following year the virus was isolated from africanus mosquitoes collected from the same forest. Antibodies against Zika virus were first identified in humans in the early 1950s. Since then, Zika antibodies have been found in several animal species, including monkeys, elephants, lions, zebras, and rodents. However, nonhuman and human primates are thought to be the major hosts and reservoirs of the virus.

Virus Characteristics

Zika virus is a member of the virus family Flaviviridae. The flaviviruses are part of a larger group called arboviruses. These are viruses that develop in arthropods (chiefly blood-sucking mosquitoes and ticks), which transmit the virus by bites to vertebrate hosts. The viruses do not harm their arthropod carriers but reproduce and cause illness in the host animal.

There are two major lineages of Zika virus: an African lineage and an Asian lineage. Each lineage contains multiple strains of the virus. Strains belonging to the Asian lineage have spread over a wide geographic range and are commonly found in several distant regions, including parts of the Americas.

Transmission

In the vast majority of cases Zika is spread through the bite of an infected Aedes mosquito—specifically A. africanus and A. albopictus. These mosquitoes generally lay their eggs in standing water, such as in ponds, marshes, pools, buckets, and even discarded car tires filled with rain or waste water. Aedes mosquitoes are aggressive daytime biters; however, they are also active at night. The mosquitoes become infected with Zika when they feed on the blood of an infected individual. The mosquito then transmits the virus to other individuals when it bites them.

Zika also can be passed from a pregnant woman to her fetus. Infection during pregnancy can cause certain severe birth defects, notably microcephaly, an abnormal smallness of the head in newborns that generally results in severe brain impairment and other developmental difficulties.

There also is evidence for transmission of Zika through sexual contact with an infected individual.The virus can be spread this way even if the infected person does not have symptoms at the time.

Symptoms

Most humans who become infected with Zika virus show no symptoms of illness. When symptoms are apparent, they generally consist of low-grade fever, rash, headache, joint and muscle pain, and conjunctivitis (inflammation of the eye). The illness typically is self-limiting, lasting about four to seven days.

Although generally mild in nature, Zika fever was associated in the early 21st century with severe neurological complications. Among these were the aforementioned microcephaly and Guillain-Barré syndrome—an autoimmune condition of the peripheral nervous system that causes muscle paralysis.

Diagnosis, Treatment, and Prevention

Diagnosis of Zika is based on a person’s recent travel history, symptoms, and—most importantly—the results of a blood or urine test. A definitive diagnosis through such tests is needed to distinguish Zika fever from dengue and other illnesses with similar symptoms. Most cases of Zika fever are self-limiting and can be treated with supportive care, such as getting adequate rest and drinking fluids to prevent dehydration.

There is no vaccine or medicine for Zika virus. The best way to prevent Zika is to reduce or prevent transmission. Using insect repellent, wearing clothing that covers the arms and the legs, and making sure that window and door screens are intact will help prevent mosquito bites. Elimination of sources of standing water, which serve as breeding sites for mosquitoes, can help to control mosquito populations.

Public Health Concerns

The spread of Zika virus from Asia is thought to have been aided by human travel and the spread of Aedes mosquitoes that transmit the virus. The principal vectors (transmitters of the virus), A. aegypti and A. albopictus, are native to Africa and Asia, respectively. However, both species have spread from those regions to distant areas, including the South Pacific and the Americas. In addition, Zika virus may be readily taken up and transmitted by endemic Aedes species. For example, A. polynesiensis, native to islands in the South Pacific, was a probable carrier in a large Zika fever outbreak in French Polynesia in 2013.

Through the late 20th century, cases of Zika fever in humans were reported only sporadically in countries in Africa. The Asian lineage of the virus emerged in Micronesia in 2007 and later expanded to other parts of Oceania and to the Americas, where it began to circulate locally. In 2015 it gave rise to a significant Zika outbreak in Brazil that produced an unusually high number of cases of microcephaly in newborns. A similar outbreak around the same time period was reported in French Polynesia.

By early 2016 Zika virus had circulated to more than a dozen regions in the Americas, including Brazil, Puerto Rico, and Mexico. The extensive and rapid spread of the virus in the Americas, along with its associated neurological complications, led the World Health Organization (WHO) to declare Zika virus a Public Health Emergency of International Concern on February 1, 2016.

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